Advances in photolithography are acknowledged to be the key to further increases in packing density for microcircuits. These in turn will result in lower costs for integrated circuits. As the feature size in photomasks becomes smaller, it becomes more difficult to exclude the effects of microscopic foreign particles on the lithographic resolution. The art has reached the point where very small dust particles are comparatively large enough to cause pattern defects. A solution to this is to use electron beam lithography because the small dust particles are transparent to electrons and will not interfere with the exposure of the pattern. However, photolithography continues to be widely used and there is ample room for continued advances in this technology.
Prior to this invention, eliminating mask defects that result from dust interfering with the optical exposure of the mask has simply been a matter of controlling the cleanliness of the processing environment. This is a conventional control measure that is used in all semiconductor processing. However, controlling particles of a size that can interfere with very high resolution photolithography is difficult and expensive.
The problem of dust particles adhering to the photomask is aggravated by the tendency of the mask to accumulate and retain static electrical charge when exposed to intense actinic radiation. The static charge attracts dust.